Colorectal cancer is the second most prevalent cancer in the United States, affecting both men and women. Until recently, the only viable treatment for this disease has been surgery, which has a poor prognosis for patients with transmural extension of tumor and metastasis to regional lymph nodes. A dramatically improved prognosis was indicated in a recently reported randomized Phase II-active specific immunotherapy trial, which showed that immunization of patients with autologous tumor cells admixed with Tice BCG (Bacillus Calmette Guerin) (Institute for Tuberculosis Research, Chicago, Ill.) significantly increased delayed cutaneous hypersensitivity responses and, over a four year period of time, significantly decreased recurrence and mortality (3).
There have been numerous publications describing the identification of colon carcinoma-associated antigens (4-9). The majority of these antigens were identified using monoclonal antibodies generated by immunizing mice with some form of the colon tumor (extracts, dissociated cells, membrane preparations, and etc.) or colon tumor cell lines. These mouse antibodies identify a repertoire of antigens that were deemed to be antigenic in the mouse. In addition to these studies there are several reports of human monoclonal antibodies that show specific reactivity with tumor material (19).
Using peripheral blood B-cells from colorectal patients actively immunized with autologous tumor cells and BCG in immunotherapy protocols, we have successfully developed a strategy for producing human anti-tumor monoclonal antibodies (1). Unlike mouse monoclonal antibodies generated against human colon cancer, which often recognize tissue components also found in healthy individuals, such as CEA, our human monoclonal antibodies exhibit no reactivity with CEA, blood group determinants or histocompatibility antigens, indicating that these antibodies are characterized by a specificity confined to those epitopes that are recognized as immunogenic in the autologous host.
We have used these human monoclonal antibodies as probes to identify tumor antigens. We have identified a particular antigen in colon tumors, extracts of colon tumor cell lines and human tumor xenografts generated in nude mice. The subject antigen is characterized by containing an epitope recognized by human monoclonal antibody (MCA) 16-88, which has been detected in approximately 60% of all colorectal tumors. We have found that the epitope identified by human MCA 16-88 is not recognized by mouse monoclonals, even those generated by immunizing mice with the human antigens containing this epitope.